The many tensions with dark-matter based models and implications on the nature of the Universe

Abstract

The current standard models of cosmology (SMoC) - specifically LCDM and warm dark matter models - served for a few decades as the basis of research in astronomy and cosmology, and have been studied extensively. However, fundamental tensions between observations and theory have emerged. This updated review has two purposes: to explore new tensions that have arisen in recent years, compounding the unresolved tensions from previous studies, and to use the shortcomings of the current theory to guide the development of a successful model. In any representative volume, more than 90 per cent of all galaxies have thin, extended star-forming ancient disks. But these structures are too fragile to withstand the repeated mergers that dark matter would induce. According to SMoC cosmological simulations, galaxies in the Local Group around the Mpc scale should be distributed in a spheroidal configuration. Observations show that they are instead arranged in thin planes. This poses major questions on the nature and dynamical history of the Local Group. Furthermore, there exist mutually correlated planes of satellite dwarf galaxies located around the Andromeda and Milky Way galaxies. These planes may have been created by the tidal forces generated by a previous encounter between the two galaxies. Also the configuration of the nearby M81 group poses challenges to the SMoC. None of these structures could exist in the presence of dark matter, because dynamical dissipation would cause the galaxies to merge within a Gyr time scale. In addition, the El Gordo galaxy cluster has been observed at a redshift at emission of z=0.87 to already have reached a mass of about 2 x 10^{15} Msun, being impossible in the SMoC. In the light of the growing evidence for large-(>few hundred Mpc)-scale inhomogeneities, we have shown that the Hubble Tension is simply caused by the matter bulk flows of the large scale structure. These observations suggest that the Universe is more inhomogeneous and that structures grow more rapidly than what is allowed by the SMoC on the Gpc~scale. Novel tensions have emerged from observations of the early Universe. For instance, galaxies of mass 10^9-10^{10} Msun have been detected in the redshift range of 10 < z < 20, indicating a faster galaxy formation than predicted in the SMoC. An independent indication for such early galaxy formation comes from the downsizing timescales of early-type galaxies and their associated rapid formation of central super-massive black holes (SMBHs). We discuss a few candidate models of cosmology from the literature, but most fail on all or a number of the above problems. Given the nature of the tensions, the real Universe needs to be described by a model in which gravitation is effectively stronger than Einsteinian/Newtonian gravitation at accelerations below Milgrom's acceleration scale. Interestingly, Milgromian dynamics (MOND) coincides with the generalized Poisson equation given by the non-linear p-Laplacian when p=3 with p=2 providing the standard Newtonian Poisson equation. A promising model that solves several of the above tensions is nuHDM. While it embraces MOND, eliminating the need for dark matter, it retains dark energy and consequently the SMoC expansion history. However galaxy formation appears to occur too late in this model, model galaxy clusters reach too large masses, and the mass function of model galaxy clusters is too flat and thus top-heavy in comparison to the observed mass function. The above mentioned evidence casts doubts on the viability of dark matter and dark energy as fundamental components of the Universe, with severe consequences. Specifically, the Hot Big Bang Theory cannot provide a good fit to the CMB power spectrum without invoking both of these components. Consequently, inflation - introduced to justify why causally-disconnected regions of the CMB should be homogeneous on a flat geometry - would also cease to be needed. The models that have been simulated to-date with these boundary conditions appear to not be able to generate structure rapidly enough to be consistent with the high-redshift JWST observations. Given all the noted anomalies, the classes of models that relax these boundary conditions should be explored.

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